There is no "evidence" that increased radon exposure at low levels results in increased lung cancer.
On the contrary, there is substantial evidence that increased radon exposure actually DECREASES lung cancer risk (the higher the radon, the lower the lung cancer risk). The definitive study on this subject is "Test of the Linear-No Threshold Theory of Radiation Carcinogenesis for Inhaled Radon Decay Products," by Dr. Bernard L. Cohen, Health Physics, Vol. 68, Issue 2, pp. 157-174.
The abstract of the above article is reproduced below.
Another article that you might find useful is "Is Radiation Good for You?" by Will Hively...

